1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus such as an electrophotographic copying machine which performs image formation by using a charger and a photoreceptor drum.
2. Description of the Prior Art
An image forming apparatus of this type includes a main charger for charging the surface of a photoreceptor drum and a transferring charger for transferring a toner image formed on the drum surface onto a sheet of paper. FIG. 1 shows a prior art device for supplying a high voltage power to the two chargers from one high voltage transformer.
In this prior art, a shield current of the main charger 2 is detected through a resistor R1, the detection value is compared with a reference value by an error detecting amplifier 9, the primary side of a high voltage transformer 7 is controlled through an inverter driving circuit 8 based on the comparison result, and a current is supplied from a secondary side to the main charger 2 to control a current flowing through the shield so that it is constant. This prior art suggests that the shield current of the transferring charger 5 may be controlled to be constant instead of controlling the shield current of the main charger 2 to be constant.
In the above-described prior art, the shield current of the charger is controlled to be constant in consideration of the fact that the ratio between the currents flowing from each wire of the chargers to the shield and the photoreceptor drum is in a predetermined relationship. However, in this prior art, the shield current of only one of the chargers is detected so that the shield current is constant, and the control is not performed with respect to the other charger.
For example, when the main charger 2 becomes dirty, the current is controlled to be constant by supplying more current in accordance with the dirt. However, since the dirt of the main charger 2 and the dirt of the transferring charger 5 are different, the current in the transferring charger 5 is a constant current which is different from a desired current value. This causes mistransfer and misseparation when a sheet 6 is separated from the drum.